Direct measurement of field effects on surface diffusion

We present here a method for quantitatively determining tip effects on surface diffusion during a scanning tunneling microscopy experiment. Using the technique of atom tracking, we measure the bias voltage and tunnel current dependencies of adsorbed Si dimer dynamics on Si(001). Throughout the range of typical tunneling conditions, the activation barrier for diffusion varies by less than 3%. We also find a striking difference between the electric-field effects on dimer diffusion and rotation, indicating the importance of transition states for this system.